assert/retract & dynamic predicates

[JoelHobson]

tl;dr - Is there a way to tell Scryer that a predicate defined in a source file can be changed via assert & retract?

I'm working through the book Adventure in Prolog, which guides you through using prolog to create a text adventure game. The book's introduced the use of asserta and retract to update the state of the game.

Here's a minimal example of what I'm trying to do:

here(kitchen).
move(Place) :-
  retract(here(_)),
  asserta(here(Place)).

When I try to use the move predicate, I get error(permission_error(modify,static_procedure,here/1),retract/1).. From experimenting, it looks like this is because here(kitchen). is defined in my source file. If I remove it from the source file, then assert it in the repl, the move predicate works correctly. I'm assuming that this means that only predicates added at runtime can be modified with & retract. Is my assumption correct?

It looks like swi-prolog has a dynamic predicate that can be used to allow retract to work in this context. Is there something similar for Scryer, i.e. is there a way to define a dynamic predicate in my source file so I don't have to set an initial one in the repl? I don't see anything that looks promising under builtins in the docs.

I realize that using assert & retract like this isn't necessarily good practice (the book makes this clear), but I'd like to follow the code examples in the book as closely as possible for my own learning.

[bakaq]

That's one of the uses of the dynamic/1 directive:

?- [user].
:- dynamic(here/1).

here(kitchen).
move(Place) :-
  retract(here(_)),
  asserta(here(Place)).
end_of_file.
?- here(A).
   A = kitchen.
?- move(bedroom).
   true.
?- here(A).
   A = bedroom.

I don't see anything that looks promising under builtins in the docs.

Maybe this is a indication that directives should also be documented somewhere (relevant: #2030). dynamic/1 is a part of the ISO standard (it is also marked as this in the SWI documentation), so it should work mostly the same for any compliant Prolog implementation. If I'm not mistaken Scryer follows the standard closely for dynamic/1, but SWI also makes it an operator so that you don't have to write the parenthesis.

[JoelHobson]

Ahh, thank you! I didn't know about the distinction between builtins and directives, but I was also doing some other things wrong. I was using dynamic(here(_)) and not starting the dynamic(here/1) line with :-.

I'm inclined to agree that the directives should be documented somewhere. It's not ideal for prolog learners to have to look at another implementation's docs or be familiar with the standard to know about them, but from reading #2030 I see that's not necessarily simple.

Thanks for the help!